Acoustically compensated microphone with adjustable acoustic resistance



Oct. 30, 1962 F. swlNEHART 3,061,690 ACOUSTICALLY COMPENSATED MICROPHONEWITH ADJUSTABLE ACOUSTIC RESISTANCE 5 Sheets-Sheet 1 Filed Sept. l5,1958 OQ Q .Q Y NQ I I I l u.:

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Oct. 30, 1962 F. swlNEl-IART 3,061,690

ACOUSTICALLY COMPENSTED MICROPHONE WITH ADJUSTABLE ACOUSTIC RESISTANCEFiled Sept. 15, 1958 3 Sheets-Sheet 2 lqooo/v 20,950

Oct. 30, 1962 F. swim-:HART 3,061,690

ACOUSTICALLY COMPENSATED MICROPHONE WITH ADJUSTABLE ACOUSTIC RESISTANCEFiled Sept. l5, 1958 5 Sheets-Sheet 5 l arent 3,061,696 Patented Oct.30, 1962 tice ACOUSTICALLY COIPESATED MICRRHONE WITH ADJUSTABLE ACOUSTICRESISTANIE Frank Swinehart, Cedar Rapids, Iowa, assignor to 'I he TurnerCompany, Cedar Rapids, Iowa, a corporation of Iowa Filed Sept. 15, 1958,Ser. No. 760,995 13 Claims. (Cl. 179-1155) This invention relates to adynamic lavalier microphone. More particularly, this invention relatesto a microphone of the type which has a diaphragm-mounted voice coilwhich moves in the field of a permanent magnet and which is constructedso as to be adapted to be worn on a neck cord from which the microphoneis suspended and positioned near the body of the user as at his chest.

The size and type of construction required for the normal manner andposition of use of lavalier microphones has in the past resulted inconsiderable ditiiculty in achieving the desired quality of audio outputfrom them. Even though an appropriate balance between high and lowfrequencies was achieved, lavalier microphones have in the past had atight or boXy quality in the sound reproduced from them. It has beenfound that this diiculty can be overcome in an otherwise properlyconstructed lavalier microphone by tuning or resonating the back cavitythereof by a vent tube `which has been found to be most effective whenits open end is near the top or front of the microphone. By this meansin conjunction with other features to be described below, it has beenfound possible to eliminate the tight or boxy quality of the sound froma lavalier microphone and to achieve the pleasing and natural soundquality desired.

It is therefore an object of this invention to provide an improveddynamic lavalier microphone.

It is a further object of this invention to provide a dynamic lavaliermicrophone wherein the back cavity is tuned by a vent tube opening tothe front of the microphone.

It is a further object of this invention to provide a dynamic lavaliermicrophone wherein the over-all frequency response is such as tocompensate for the fact that the microphone is used near the body.

Other objects, features, and advantages of the present invention will bemore fully apparent from the followingy detailed description taken inconnection with the accompanying drawings in which like referencecharacters have been used to refer to like parts throughout and wherein:

FIGURE 1 is a longitudinal sectional view showing a lavalier microphoneconstructed and assembled in accordance with the present invention.

FIGURE 2 is an end View of the front or top of the microphone of FIGUREl.

FIGURE 3 is a detailed sectional view of a member inserted in the caseof the microphone of FIGURE 1 to form the vent tube.

FIGURE 4 is a detailed sectional view of the interior or voice coil andmagnet assembly of the microphone shown in FIGURE 1.

FIGURE 5 is a plan view of an inner grille member included in themicrophone of FIGURE 1.

FIGURE 6 is a schematic wiring diagram of the microphone of FIGURE l.

FIGURE 7 is a graph of the frequency response of the microphone ofFIGURE l wherein frequency in cycles per second is plotted as abscissaand response in decibels is plotted as ordinate.

FIGURE 8 is an exploded view of the component parts of the microphone ofFIGURE 1.

Turning now to the drawings, there is shown in FIG- has mating exteriorthreads on an indented shoulder thereof. The case member 10 has a top orfront section Itia of enlarged cross-sectional diameter on the interiorof which is formed a first shoulder 10b and a second shoulder 10c. Itwill be noted from FIGURE l that the case member terminates at its frontor top end in a lip or ange member 10d which projects forwardly and thatthe first shoulder member 10b extends inwardly from this ange member10d. The portion 10a of the case member between the first shoulder 10band the second shoulder Itis has an interior diameter which isintermediate between that of the interior diameter of flange 10d and ofthe main portion of the case. The shoulder 10c extends inwardly fromthis intermediate diameter and merges with the main portion of the casemember.

Seated on the second shoulder 10c is an insert member 12 as shown inFIGURE l and as shown in greater detail in FIGURE 3. Insert member 12 isa round sleeve of metal having a spiral groove 13 cut on the innersurface thereof. The spiral groove 13 terminates in an inlet 13a whichis positioned in the forward or front part of the casing section 10a andin an outlet 13b positioned adjacent the second shoulder in the casing10. The metal sleeve 13 carrying groove 13a has an outer diameter equalto the inner diameter of the enlarged section 10a of the casing 16 andseats against the second shoulder 10c in the casing.

A tubular sleeve member 14 having an outer diameter equal to thesmallest inner diameter of the sleeve member 1.2 is tightly fitted orpressed inside of the sleeve member to close the spiral groove in thesleeve member and thereby form the vent tube. It will be noted that thetube 14 is longer than the sleeve 12 and projects forwardly of it. Theexterior surface of this forwardly projecting portion of the tube 14 isthreaded as at 15 and receives in threaded engagement therewith a frontframe member I6 which projects forwardly of the flange 10d on casing It?and terminates in an inwardly projecting shoulder portion 16a. Restingin back of the shoulder 16a is the outer grille 17 bearing a name plate18. In back of the outer grille an inner grille 19 is positioned insandwiched relation between a pair of sealing washers 20 and ZI. Theinner grille 19 is shown in plan view in FIGURE 5 and comprises acircular plate having a plurality of openings therein as shown.

In back of the sealing washer 21 is positioned the interior assemblywhich for clarity of illustration is indicated by the general referencecharacter 22 in FIG- URE 1 and is shown in greater detail in FIGURE 4.

Interior 22 comprises a magnet 23 having a pole piece 24 which ismounted in a yoke 25 by means of a machine screw 26. A grounding lug 27is conveniently seated under screw 26 whereas other lugs such as 30 maybe attached to the yoke by screws 28, 29 and insulating member 31. Itwill be noted that a portion of the pole piece is exteriorly threaded asat 32 and receives a retaining ring 33 which is interiorly threaded tobe mounted thereon. A felt ring 34 is mounted between retaining ring 33and a stator plate 35 which is shaped to iit over the end of yoke 25 andhas a central opening through which the magnet pole protrudes. Theannular space 36 between the edges of this opening and the magnet polereceives the voice coil 37 which, for clarity of illustration is shownin FIGURE 4 in disassembled position. Voice coil 37 is attached todiaphragm 38 which is supported by diaphragm ring 39 which in turn restson stator plate 35. The diaphragm ring 39 may conveniently be affixed tothestator plate by means of a bead of cement around the outer diameterof the diaphragm rmg.

As may be seen in FIGURE l, the stator plate and diaphragm assembly areheld in supported relationship against sealing washer 21 by means ofanother sealing washer 40 which abuts on one side against the end oftube 14 and on the other side underneath the shoulder formed by thestator plate 35 projecting down over the yoke 25. Front member 16threadedly engages tube 14 to retain this assembly in position.

The Lback cavity of the microphone is formed by the net enclosed volumebehind the sealing washer 40. It is important that all mechanical jointsbe tight in that leakage will impair the effectiveness of the resonanceestablished between this back cavity and the vent tube formed by groove13. The outlet 13b of vent tube 13 opens to the back cavity through thespace between tube 14 and shoulder i100. The inlet 13C of vent tube 13opens to a passage formed between the inner surface of the portion 10aof the casing 10 and the outer surface of the front support member 16which is threadedly received in position on tube 14 after the interiorassembly has been positioned on sealing washer 40 on the end of tube 14.The end of this passageway from the vent tube is covered by a ventgrille 41 which seats in the first shoulder 10b of the outer casingmember and is provided with a central aperture to snugly receive thefront member `16. The sequence and mode of assembly of the parts may beseen more clearly by reference to the exploded view of FIGURE 8.

A transformer 42 4is positioned in case 10 in back of the interiorassembly 22. The transformer may conveniently be mounted in the case bysponge rubber packing `43 which surrounds it to prevent shock andvibration. The forward wall 43C of this packing defines the rear wall ofthe back cavity. The space between the transformer 42 and the rear ofthe interior `22 which extends substantially down `to the secondshoulder 10c is preferably packed lightly with spun ber glass insulation44. Such spun fiber glass insulation `44 is preferably also lightlypacked in back of `the vent grille 35 down to the inlet end of thespiral groove vent tube 13.

The electrical connections to and from the voice coil transformer 42 tothe microphone and shielded cable 45 are conventional and are shown inthe assembly view of FIGURE l and in the schematic wiring diagram ofFIGURE 6. The cable 45 may enter the plug member 11 .through rubbertubing `46 held in position by a grommet y47 which in turn is held inplug 11 by a pressure plate 48 attached to the plug 11 by any convenientmeans such as screws to which lugs may also be attached as shown. Y

Referring `to FIGURE 8 it will be noted that the shield of cable 45 isdirectly connected by a wire 50 to the center lug 30 on the interior 22thereby grounding its yoke. The three conductors 1, 52 and 53 of thecable 45 are respectively connected to the two ends and the center tapof the primary winding `of transformer 42, The secondary Winding oftransformer 42 is of course connected by wires y54 and 55 to lugs on theinterior assembly which in turn are connected to the voice coil in theusual manner.

In use, the microphone may be positioned in any convenient holder towhich a neck cord may be attached to support `the microphone near thebody of the user. The enlarged portion a of the casing terminating inthe narrowing section which forms the shoulder 10c aords on the exteriorof the casing a convenient shoulder or stop means which permits themicrophone to be inserted and retained in such a holder. The over-allfrequancy response of the microphone is selected to help compensate forthe fact that the microphone is to be used near the body. A preferredexemplary frequency response is shown in the graph of FIGURE 7 whereinfrequency in cycles per second is plotted as abscissa against responsein decibels plotted as ordinate. In general terms, it may be stated thatthe desired frequency response is a smooth curve which has the mediumand high frequencies from four to six decibels above the lowfrequencies. In particular, the cavity in back of the interior, which,to a large extent, controls the =low frequency response of the unit(below 200 cycles) is dimensioned so as to be resonant with the venttube at approximately 70 cycles. Furthermore, the open end of the venttube is arranged in such a ways as to take best advantage of itsfunctioning. That is to say, the open end is positioned near the frontof the unit and the low frequency sound enters the microphone primarilythrough this vent tube. Extensive tests have shown that even though thebalance between the high and the low frequencies shown in FIGURE 7 wasachieved, `the tight or boxy quality of prior art lavalier microphonespersists if the vent tube is not used. It was only after the back cavitywas tuned by a vent tube (in one exemplary embodiment having across-sectional area of approximately 0.014 square inch and a lengthalong the spiral groove of approximately 8 inches) that the desiredpleasing and natural quality of sound was achieved. Experiments havealso shown that the vent tube is most effective when its open end isnear the top of the microphone.

In operation, the sound in addition to directly actuating the diaphragm39 and voice coil 37 also passes through the vent grille `41 and entersthe inlet 13a of the vent tube 13. The sound is carried down the rest ofthe length of the vent tube spirally arranged around the interior 22 andenters the back cavity through the outlet 13b of the vent tube near theshoulder 10c and behind the diaphragm. The purpose of the spun glass `44and 44a is to smooth out the frequnency response of the unit.

The resonant frequency of the back cavity is determined bythe enclosedvolume -of the cavity and .the length and crosssection of the vent tube.This relationship may be expressed in terms of the electrical equivalentas follows:

Fres.:

The resonant frequency of the cavity-vent tube system can also bedetermined by this same relationship; the L terms being determined bythe length and cross-sectional area of :the vent tube and the C termbeing determined by the cavity.

It has been experimentally found that if the interior 22 is mounted in a'sealed cavity, that is, Without a vent tube to resonate it, thefrequency response will show that the microphone starts to llose outputat about 300 cycles. As the frequency is lowered, the response continuesto fall so that the quality of the sound is impaired. The resonant backcavity-vent tube system overcomes this defect. Furthermore, if theinterior 22 is mounted as shown in FIGURE l, but without the inner`grille 19, the response will show a serious loss above about 4000 or5000 cycles. It is, then, the back resonant cavity-vent tube system, theinterior itself, and the inner grille which determines over-allperformance of this microphone. The external shape also has an influenceupon this performance. Fur lthermore, by adjusting the retainer ring 33shown in FIGURE 4 which compresses the felt ring 34, it is possible toinfluence the microphone output. Tightening this ring .tends to lowerthe response below 1000 cycles with respect to the higher frequencies.Adjustment of this retaining ring is the last step in finallydetermining the desired response for the microphone.

rI`he frequency response shown in FIGURE 7 achieved by a microphonehaving the above enumerated GQ11811116.-

lavalier microphone near the body of the user.

While a preferred exemplary embodiment of the present invention has beendescribed in detail above, it -will be understood that modifications andvariations may be effected therein without departing from the spirit andscope of the novel concepts of the present invention as defined by thefollowing claims.

I claim as my invention:

`l. In a small compact microphone of the dynamic type 'having a movablevoice coil mounted on a diaphragm substantially closing, and resonatingwith, a rear cavity, all in a casing having an open front, means,including an outer grille over the open front of said casing and saiddiaphragm and a grille-like bafile disk between said outer grille andsaid diaphragm, said baffle disc having perforations therethrough of 4asize to selectively increase the high end frequency response of thefrequency response curve of said microphone.

2. In a `dynamic microphone of the diaphragm-voice coil type, theimprovement to better the reproduction of tonal quality comprising meansforming a cavity positioned to the rear of, .and communicating with theback of, said diaphragm and a vent tube connecting said cavity to thefront sound field adjacent to said diaphragm by an annular openingaround said diaphragm sonic damping means in said annular opening, meansother- Wise substantially sealing said cavity, said cavity and said venttube together being resonant to increase the lower end frequencyresponse of said microphone.

3. A dynamic microphone comprising, a generally cylindrical outer casehaving a fron-t opening, said casing enclosing inner and outer sleevemembers, a back cavity at least partly in said sleeve members, said ventpassage opening at one end to said back cavity inside said case memberto be resonant therewith at low frequencies and opening at the other endto the outside front of said case, a permanent magnet assembly in saidsleeve members having a central pole and a surrounding annular poleradially spaced therefrom to define an annular'air gap, a ring offibrous material between said outer annular plate and a retaining ringadjustable along said inner pole, a diaphragm mounted voice coil free tomove said air gap, said diaphragm being operatively connected to saidannular outer pole, said retaining ring being axially adjustable tovariably compress said fibrous material to change its sonic resistanceand thus adjust the frequency response of said microphone.

4. Apparatus as in claim 3 wherein fibrous material is lightly packed inat least a portion of said resonant system formed by said back cavityand said vent tube to smooth out the frequency response curve of saidmicrophone.

5. A microphone of the diaphragm-voice coil type comprising a diaphragm,a magnet having an axially ex- ,tending central pole piece means forminga vented, resonating cavity positioned to the rear of said diaphragm, arestricted and transversely extending passage means connecting the backof said diaphragm to said vented cavity, said cavity being resonant withsaid diaphragm to increase the lower frequency response thereof, sonicfriction producing means to control the intermediate frequency responseof said microphone comprising a layer of sonic friction producingfibrous material in, and extending across, said passage means, and means`to variably tune said intermediate frequency response of saidmicrophone comprising a wall means adjustably movable axially along saidelongated central pole piece means to variably compress said layer offibrous material, said movable wall means defining one side of saidtransversely extending passage means.

6. A microphone of the `diaphragm-voice coil type cornprising adiaphragm, an elongated, small diameter vent tube means opening todirectly receive low frequencyV sound separately from said diaphragm,means at the rear of said diaphragm forming a cavity vented by andresonant with said Vent tube means and communicating with a smallervolume yat the back of said diaphragm by a restricted passage means toincrease the lower frequency response of said diaphragm, means tocontrol primarily .the intermediate `frequency response of saidmicrophone comprising sonic friction producing, fibrous material in saidrestricted passage means 4and an adjustably movable wall means variablydefining said restricted passage and engaging and variably compressingsaid fibrous material.

7. In a microphone of the dynamic diaphragm-voice coil type yhaving adiaphragm mounted voice coil movable in the air gap of its permanentmagnet mounted in back `of said diaphragm, Ithe improvement to betterthe tonal quality and the response curve of said microphone comprising agrille-like restricting member perforated to increase the upper endfrequency response of said microphone and mounted outwardly from thefront of said diaphragm, means forming an enclosed cavity in back ofsaid diaphragm, and means forming a long, small crosssectional area venttube extending around, and connecting sa-id cavity to the periphery ofthe front of, said microphone, said cavity being resonant with said venttube to increase the lower end frequency response of said microphone.

8. A dynamic microphone comprising a permanent magnet unit having acentral pole and an outer annular pole portion to define an annular airgap therebetween, aretaining ring adjustable aolng said cen-tral pole, aring of fibrous material on said central pole adjustable clamped betweensaid annular pole portion and said retaining ring, a diaphragm securedrelative to said annular pole portion, a voice coil mounted on saiddiaphragm forl axial movement in said air gap, means :to control thehigher frequency response of said -microphone including a perforatedgrill-like member in front of said diaphragm, an outer grill memberprotectively covering said perforated member and said diaphragm, saidmagnet-diaphragm assembly being mounted in a case having an opening inthe front thereof, an enclosed cavity in said case and in back of saiddiaphragm, means forming a long vent tube passage helically surroundingsaid magnet-diaphragm assembly in said case, said vent passage openingat one end to said enclosed cavity and opening iat its other end to theoutside of said case, said cavity being resonant with its vent passageat lower frequencies to increase the lower end response of lsaidmicrophone, said retaining ring being adjustable in position to adjustand tune 'the intermediate frequency response of said microphone.

9. A dynamic microphone comprising, an outer case having an opening atthe front thereof, an outer sleeve member positioned within said casenear said front opening, an inner sleeve member supported within saidouter sleeve member, one of said sleeve members having a helical groovetherein defining with said other sleeve member a long vent passage ofsmall cross-sectional area, a permanent magnet unit having an air gapand mounted within said inner sleeve member, a diaphragm mounted voicecoil free to move in said air gap, means to support said diaphragm infront of said sleeve members, a cavity in back of and substantiallyenclosing said diaphragm, said vent passage opening at one end to saidenclosed cavity and at its other end to the front portion of said case,said cavity being resonant with said vent passage -to increase the lowerend frequency response of said microphone. i

'10. A dynamic microphone comprising a permanent magnet assembly havinga central pole and a surrounding outer annular pole portion spacedtherefrom to define an annular air gap therebetween, a retaining ringthreadedly received on said central pole, a ring of fibrous material onsaid central pole between said adjustable ring and said annular poleportion to dene an adjustable sonic friction passage, adiaphragrnsecured :relative to said outer pole portion, a voice coil mounted onsaid diaphragm for axial movement in said air gap, an inner grill-likemember perforated to increase the higher frequency response of saidmicrophone and mounted in front of said diaphragm, an outer grill memberprotectively covering said perforated member and said diaphragm, saidmaginet-diaphragm assembly being mounted in a case having an topening inthe front thereof, a cavity in said case and in back of, andsubstantially close by, said diaphragm, means forming a vent passagehelically surrounding said magnet-diaphragm assembly in said case, saidvent passage opening :at one end to said enclosed cavity and opening atthe other end thereof to said `opening in the front of said case, saidcavity being resonant with its vent plassage at lower frequencies toincrease the lower end response of said microphone, said retaining ringbeing adjustable in position to adjust the intermediate frequencyresponse of said microphone, and fibrous material lightly packed in atleast a part of said resonant system formed by said vent tube and saidenclosed cavity to smooth ont the frequency response thereof.

ll. A microphone comprising a diaphragm means forming a ventedresonating cavity positioned to the rear of said diaphragm, passagemeans connecting the back of said diaphragm to said cavity, said cavitybeing resonant with said diaphragm to increase the frequency response ofsaid microphone below approximately 200 c.p.s., said passage meansincluding a chamber of smaller volume than said cavity positioneddirectly back of said dilaphragm, said chamber forming a resonating partof said cavity, means to better the intermediate frequency response ofsaid microphone below approximately 1,000

cps. comprising sonic friction producing, porous mate.

r-ial in said passage means between said chamber and said cavity, meansto better the tone or quality of said microphone and increase itsfrequency response above approximately 5,000 c.p.s. comprising bafflemeans positioned forwardly of the front of said diaphragm.

12. A microphone comprising a sound vibrated diaphragm having its frontexposed to the front sound field, means forming a resonating cavitypositioned to the rear of said diaphragm, passage means connecting theback of said diaphragm to said cavity, said cavity being [resonant withsaid diaphragm at a frequency of approximately 300 c.p.s. and meansforming an elongated passage venting said cavity to outside of saidmicrophone through sonic damping means, said elongated passage having `asmall cross-sectional area to attenuate high sonic frequencies andhaving a length to resonate with said cavity and said diaphragm atapproximately t c.p.s.

13. In a dynamic microphone of the compact small iavalier-type having adiaphragm mounted voice coil free to move in the air gap of a permanentmag-net unit posiyti'oned in back of said diaphragm, the improvementcomprising, an enclosed cavity positioned to the rear of, andcommunicating with the back of, said diaphragm, cylindrical meansforming a long and small cross-sectional area vent tube in a helix, saidcylindrical means being positioned around said magnet, an inlet to saidvent tube positioned at a front peripheral edge of said microphone andadjacent said diaphragm, an outlet from said vent tube being in opencommunication with said cavity, said cavity being otherwiselsubstantially sealed, said cavity and said vent tube being resonant toincrease the lower end frequency response of said microphone.

References Cited in the le of this patent UNITED STATES PATENTS1,847,702 Thuras Mar. 1, 1932 1,964,606 rfhuras June 26, 1934 2,401,328Black June 4, 1946 2,847,326 Muller Aug. l2, 1958 2,848,561 Gorke Aug.19, 1958 2,862,070 Bonnafous Nov. 25, 1958 2,865,464 Gorike Dec. 23,1958 2,939,922 Gorike June 7, 1960 FORETGN PATENTS 873,259 Ger-many Apr.13, 1953 183,462 Austria Oct. l0, 1955 1,014,595 Germany Aug. 29, 1957

